Manhole structures are arranged at predetermined intervals along sewage lines, for example, for inspection and cleaning purposes, and for providing an intersection between two branches of a sewer line merging into a third, to name just a few applications.
A manhole structure is typically comprised of a manhole base, an intermediate section and a top riser section. The manhole based is placed beneath grade at a depth dependent upon the depth of the sewer line entering the base, through an opening made water-tight through a cooperating gasket. The intermediate and riser sections are utilized to bring the manhole structure up to grade, and provide a water-free, hollow interior which may be easily entered for repair and/or maintenance. The open top of the manhole structure is sealed with a manhole cover removably supported upon a supporting frame, which either rests directly upon the top of the riser section or rests upon the top surface of a grade adjustment ring which may, for example, be one of a plurality of grade adjustment rings positioned between the top surface of the riser section and the bottom surface of the manhole cover support frame, in order to bring the frame opening supporting the manhole cover up to grade.
In order to prevent subsurface water from entering into the interior of the manhole structure through the region of the engaging surfaces between the manhole cover and the adjustment rings, and/or between the engaging surfaces of adjacent adjustment rings, and/or between the engaging surface of an adjustment ring in the top surface of the riser section, mortar is placed between these engaging surfaces. However, due to irregularities in the engaging surfaces; shrinkage and dynamic loading, natural erosion and deterioration of the mortar due to frost heave, surface water eventually seeps through the above-mentioned regions to enter into the interior of the manhole structure.
It is further desirable to install manhole structures in such a way as to limit on-site activities to a minimum and it is, therefore, desirable, not only to provide water-tight seal in the region between the manhole riser section and the manhole cover support frame, but to be able to accomplish this through the use of a manhole structure, which is of simplified design and is easy to install and does not require additional, complicated manual activity at the installation site.